This invention relates to chromium-containing olefin polymerization catalysts.
Efforts have been made for some time to prepare catalysts utilizing chromyl chloride (CrO.sub.2 Cl.sub.2). Chromyl chloride can be reacted for instance with a substrate such as silica containing a large number of closely spaced (paired) OH.sup.- groups so that each chromium becomes attached to the surface through two oxygen links. ##STR1## Silicas calcined at 400.degree. C. or below provide such a support. However, these materials exhibit poor activity as olefin polymerization catalysts due to the low calcining temperature which leaves a high background OH.sup.- population. A secondary calcining step in air at 800.degree. C. yields an ordinary Cr/silica catalyst, well known in the prior art, giving good activity, a high MI potential, and a relatively narrow molecular weight distribution.
On the other hand if the silica substrate is first calcined at a high temperature such as 800.degree. C., and then treated with chromyl chloride, a relatively inactive catalyst still results. This time only widely spaced OH.sup.- groups remain (singles) and these react to yield the monochloride species, attached to the surface through only one oxygen link. ##STR2## Apparently the chloride in this species renders it inactive. Again a secondary calcining step at 800.degree. C. provides ordinary prior art catalysts by burning off the chloride.